Such a hydraulic unit is derived from the not previously published DE 10 2007 054 376 A1. In the case of the hydraulic unit proposed in that document, all of the essential components required for the hydraulically variable transmission of cam lobes to the gas-exchange valves and the pressure chambers are assembled in a common hydraulic housing in a sandwiched construction. The bottom part of the housing has a very compact structural configuration and the middle part of the housing involves an essentially flat plate, so that each of the medium-pressure chambers is limited to a correspondingly small volume.
As explained in the cited publication, however, a small-volume medium-pressure chamber can be problematic during the starting procedure of the internal combustion engine, especially if it involves a starting procedure at low outside temperatures and when the internal combustion engine has been at a standstill for a long time. This is based on the fact that, during the starting procedure, the hydraulic medium supply system of the internal combustion engine is still feeding an insufficient flow of hydraulic medium into the medium-pressure chamber and only the hydraulic medium volume that remains in the medium-pressure chamber and that also contracts at low temperatures is an insufficient amount for completely refilling an expanding high-pressure chamber. This problem applies to greater degrees for starting procedures repeated within a short time sequence, because in this case, the hydraulic medium consumption from the medium-pressure chamber can be larger than the volume fed back from the hydraulic medium supply system of the internal combustion engine. Such multiple starting procedures are typical, for example, for taxis at taxi stands.
For solving these problems, in the cited publication it is proposed to form in the top part of the housing a low-pressure chamber used as a hydraulic medium reservoir that is connected to the medium-pressure chamber via a throttle opening in the middle part of the housing. With the help of the low-pressure chamber, first, the hydraulic medium reservoir required during the starting procedure of the internal combustion engine expands for the medium-pressure chamber and consequently for the high-pressure chamber and, second, the risk of suction of gas bubbles is largely eliminated. The latter is realized by the middle part of the housing that separates the low-pressure chamber from the medium-pressure chamber, so that, during the standstill phase of the internal combustion engine and with this cooling and consequently contracting hydraulic medium, the formation of gas bubbles in the medium-pressure chamber is prevented by the feeding of hydraulic medium from the low-pressure chamber.
One disadvantage, however, is the expense for producing such a throttle opening in the form of the very small diameter of a stepped borehole equal to only a few tenths of a millimeter through the middle part of the housing. For example, in the case of a borehole produced with cutting, high tool wear or frequent tool failure is to be taken into account, while production by laser beam leads to undesired high form and cross-sectional deviations from the desired geometry of the throttle opening.